1. Field of the Invention
Aspects of the present invention relate in general to electrical resonators. Aspects include a substrate containing a micromechanical acoustic resonator comprising a primary portion and an enlarged portion. Additional aspects include a low-cost method of manufacturing the resonator for accurate frequency in a fixed-frequency oscillator application.
2. Description of the Related Art
An electrical network consisting of a resonator (e.g., electrical, electromechanical, and electromagnetic resonators) can be used as a frequency reference for electrical systems. A frequency reference determines the oscillation frequency in an oscillator loop by providing a stable frequency at which the phase shift in the loop is zero (or an integer multiple of 2π). Considering an oscillator to have three parts; the gain stage, the feedback network, and auxiliary components; the oscillation frequency is largely determined by the phase shift in the feedback network.
Electromechanical or “acoustic” resonators are a popular choice for the feedback network because of their phase characteristics. Acoustic resonators can provide excellent frequency stability because they can attain excellent quality factor Q. A high Q provides a high gradient of phase over frequency (i.e., a sharp phase transition). Acoustic resonators are enclosed in a cavity for long-term stability and performance.
The zero-phase frequency of the oscillator is closely dependent on the resonant frequency of the network. (A resonant frequency is a frequency at which the phase shift is zero and the impedance is low. At an anti-resonant frequency, the phase shift is zero and the impedance is high.) The resonant frequency of the network is dependent on the resonant frequency of the constituent resonator.
In the activity of manufacturing the constituent resonator, variations will be observed in the resonant frequency. The extent of these variations is one culprit of high manufacturing cost. Large variations are not resolvable and reduce the manufacturing yield. Moderate variations must be reduced to acceptable tolerances through additional processing activities. Such activities are essential and costly.